Corrosion inhibition



"2,962,356 CORROSION INHIBITION Iral B. Johns, Dayton, Ghio,;assigno'r to- Monsanto t l hemical Company, St.-Louis, Mo.,-a corporation of Delaware Nonrswia FiledSept. 9, 1-953,Ser. No. 379,283 '5 Claims. Cris- 139 solved therein.

Metallic titanium is becoming an increasingly important material of construction, particularly because of its high strength to weight ratio and its resistance to certain types of corrosion, such as corrosion by sea-water or other salt solutions. One of the disadvantages of the metal is, however, that it is subject to acid attack, particularly by strong acids, such as the mineral acids, e.g., hydrochloric and sulfuric acids. It is, therefore, a general object of the present invention toprovide a novel'method of inhibiting th'e'attackby'acids upon metallic titanium.

While the inhibition or acid attack upon metallic titanium is obviously very useful in situations involving titanium equipment which is handling acids or acidic solutions, there is another and possibly more important utility for the present invention in connection with variousprocesses for producing metallic titanium. For example, many of the processes whichhave been proposed for the production of metallic titanium are based upon a reduction of titanium tetrahalide, such as titanium tetrachloride,

with a reducing metal such as sodium, magnesium, etc. In all of such processes, the metallic titanium is formed in the presence of a by-product halide salt of the reducing metal, so that there is a problem of separating the product metal from the byproduct salt. At the present timethis separation is generally carried out by evaporating the salt from the metal in a high temperature vacuum operation' which is, at best, a very expensive operation.

Since the by-product salts formed in the reduction of titanium tetrahalide with a reducing metal are generally relatively Water-soluble, it might appear at first glance that a simple and expedient way to separate said salts from the metallic titanium would be merely to leach the mixture of metal and salts with Water to dissolve the salts and leave the metal. As a practical matter, however, the reaction mixture resulting from such a reduction of titanium tetrahalide always contains an appreciable proportion of either or both of (1) unreacted reducing metal or (2) unreduced tetrahalide or partially reduced lower halides. Any unreacted reducing metal in the reduction products Will react very rapidly with the Water used in leaching and will form a basic solution which will then react with any titanium halides present to form waterinsoluble oxygen containing titanium compounds which will contaminate the metallic titanium. Even in the absence of excess reducing metal the titanium halides will hydrolyze in neutral water solutions to form the insoluble oxides or oxychlorides of titanium. Consequently, neutral water solutions are not satisfactory for leaching the 'saltsfrominixtures resulting from the commercial reduction of titanium halides 'With reducing --metals.

' attack.

2,952,356 Nov. 29, 1960 g The aforementioned difiiculties resulting from the'use of neutral or-alkaline leaching solutions can be avoided by utilizing acidic leaching solutions. Any base formed by [reaction "of excess reducing metal is quickly neutralized by the acid in the solution-and in acidic media any titanium halide will dissolve (rather than hydrolyze to form insoluble oxygen-containing products). While the use of acidic leaching solutions overcomes the difficulties resulting-from the use ofneutral or basic leaching solutions, ithas not heretofore been practical to use acidic solutions-because 'of the high degree of attack by acids upon metallic titanium. This attack is particularly rapid because the rrietal is generally in a rather finely divided powdered or granular form having a large ratio of surface to volume, so that rather high proportions of the tion to provide'such a method.

It has now been found that the use of an organic nitro compound as an'inhibitor of acid attack upon metallic titanium is a particularly'eifective means of reducing said It'has been further found that, unlike some inhibitors which have been suggested for use with metallic titanium, 'the inhibitors of the present invention are partichas further been found that aqueous acid solutions containing minor amounts of one -or more of said organic nitro compounds are outstandingly effective for leaching inorganic salt byproducts from mixtures of said salts with metallic titanium.

The nitro-substituted organic compounds which are useful according to the present invention can be either nitro-substituted cyclic compounds (heterocyclic or carbocyclic) or nitro-substituted aliphatic corn-pounds. The carbocyclic compounds can be either alicyclic or aromatic. The aliphatic and alicyclic compounds can be either saturated or unsaturated.

A preferred class of nitro-substituted compounds of the present invention is the class of nitro-substituted aromatics, and particularly monocyclic aromatics, such as nitrobenzene. Other examples of members of this preferred class of 'nitro compounds are the nitro-substituted alkylbenzenes, such as the nitrotoluenes, nitroxylenes, nitromesitylenes, nitroethylbenzenes, nitrocumenes, nitrocymenes, nitrostyrenes, etc., in which the nitro group is attached to a carbon atom of the aromatic ring. Other aromatic hydrocarbons, the nitro derivatives of which are suitable for use as inhibitors according to the present invention are the various polycyclic aromatics, fused-ring or otherwise, such as naphthalene, diphenyl, etc.

Examples of suitable non-aromatic carbocyclic nitrosubstituted organic compounds are nitrocyclohexane, nitrocyclopentane, l-nitrocyclopentadiene, and the various hydrocarbyl (especially alkyl) substituted derivatives thereof.

Examples of suitable nitro-substituted heterocyclic organic compounds are the various nitrofur'a'ns, nitrothiophenes, nitropyrans, and the like.

Suitable nitro-substituted aliphatic organic compounds are the nitroalkanes, e.g., nitromethane, nitroethane, nitro-iso-propane, nitro-tertiary-butane, nitro-npentane, and higher homologues, particularly those having fewer than 15 or 20 carbon atoms therein. Nitro-substituted alkenes such '3-nitro-propene-l and nitro aliphatics containing aroi'natic substituents, e.g., ifi nitroethylb enzene, are also suitable. I

The present nitro-organic compounds include not only the aforementioned compounds, or classes thereof, but also such compounds containing one or more additional nitro groups, or one or more other negative polar groups,

e.g., halogen (especially chloro), sulfonic acid or metal since, as will be seen from the examples which follow hereinbelow, the inhibitors are also effective at higher temperatures and, of course, can be utilized at lower temperatures as well.

sulfonate, hydroxyl, aldehyde, carboxyl, oxo, amide, etc., 5 The time required to leach the salts from the metallic groups or radicals. Examples of such multi-substituted titanium may run anywhere from a few rmnutes to sevorganic compounds are tetranitromethane, nitrochloroeral hours, depending upon the proportion and typeof benzenes, nitrobromoform, nitrobenzyl chloride, nitrosalt to be leached, concentrat on of acid in the leaching benzene sulfonic acid, sodium nitrobenzene sulfonate, solution, temperature of leachlng operation, physical and nitroethanol, nitropentane-diol, nitrophenol, nitrobenzyl chemical states of the salt and metal, the contacting efalcohol, nitronaphthol, nitrobenzaldehyde, nitroacetic ficlency of the solution with the solids, etc. acid, nitrobenzoic acid, nitrocinnamic acid, nitrobenz- The following results are presented as illustrat ve of amide, nitroacetophenone, nitroanisol, nitrophenetole, the hi y efiecilve llafllfe P the i 6 lllhlbliols nitrobenzal chloride, nitro phthalic acid or salts or esters of the pres t t lllvelltlon, 111 Tedllcmg acid attack p thereof, nitroresorcinol, nitrochlorophenol, nitro salicyclic m tal l titaniumacid or salts thereof, nitrophenol sulfonic acid, nitro- EXAMPLE benzoyl clllorldegand others- I Numerous nitro-organic compounds were tested for The concFntl'atlons P 'f P1' eSent mhlbltors to be used their ability to inhibit acid corrosion upon metallic any Pamcular apphcatlon depend somfiwhat upon titanium. In making these tests a weighed sample of faotorssuch as temperature, acid concentrations, degree powdemd (20 60 mesh) metallic titanium was placed t and P e and concentrau? in 100 cc. of a water solution of hydrochloric acid, to forelgn mammals Such as morgam? salts etcw but W111, which solution there was added to a measured amount of f range from about Welght Parcel!E to about a nitroorganic inhibitor. In some cases one gram of 10 Welght PeTcent, f Preferably from about Weight sodium chloride ws also added in order to determine the perceflt to about Welgnt P a {P the amount elfect of the inhibitor in the presence of an inorganic of aclfil (Q Solutlon thereof) to be P P i salt. The metal was kept in contact with the acid for a f h P t lnhlbltors are usefu specified length of time, while maintaining the acid at a i inhibiting and corrosion of metallic tltanmm under substantially constant temperature. Any appreciable atn e clrcumstances, but p y useful ln tack by the acid upon the metallic titanium resulted in the {Ilakmg P P aqueous 9 1 2; Solutlofls removlflg formation of a deep violet color in the solution (formalnorganlc Salts, especlally halldes, Chlofldes, from tion of titanium trichloride by reaction of hydrochloric mixtures of such salts with powdered or granular metalid with titanium) h degree of i attack was lic titanium. These acid leaching solutions generally termined by measuring the loss in weight of the titanium comprise from about 1 to about 25 weight percent, and sample. The results of these tests are summarized in preferably from about 3 to about 10 weight percent, of the following table.

Table Inhibitor Acid, Percent Wt. NaCl Time, 'Iemp., Ti, Wt. Remarks Cone, Percent Hrs. 0. Loss Species Wt.

Percent None 14 No.--. 1 70 13. 2 Solution turned deep violet color.

5 Yes. 18% 30 5. 6 Do. 5 Yes- 24 30 5. 2 Do.

0.1 14 No--.. 1 70 0.4 Solution remained colorless. 0. 1 14 Yes... 1 70 16. 3 Solution turned deep violet. 0.5 14 No.-.. 1 70 0.5 Solution remained colorless. 0. 5 14 Yes... 1 70 0.4 Do. 0.2 14 Yes... 1 70 0.6 Do. 0.5 5 No---. 16 30 0.2 Do. 0 0.5 5 Yes... 16 30 Do. 1-Nitro-n-butane 0. 5 5 N 0-..- 1G 30 0.3 Do. Do 0.5 5 Yes... 16 30 0.4 Do. o-Nitrochlorobenzene 0. 5 5 Yes. 24 30 0. 2 D0. p-Nitrophenol 0. 5 5 Yes 24 30 0.3 Do. o-Nitrobiphenyl O. 5 5 Yes... 24 30 0.2 Do.

1 Sample lost during filtration prior to final Weighing. an acid in a water solution. It is, of course, readily ap- I claim:

parent that any one of numerous acids would be suitable for this use, since the primary purpose of the acid is merely to supply hydrogen ions to the solution. However, the mineral acids, and particularly those such as hydrochloric and sulfuric acids, are especially suitable because they are substantially completely dissociated in water solutions and are inexpensive and readily available. To these aqueous acid leaching solutions is added one or more of the inhibitors of the present invention in amounts within the range of concentrations indicated above. For a mineral acid solution of from about 3 to 10 weight percent a particularly suitable amount is between 0.1 weight percent and about 2 weight percent, based upon the weight of aqueous acid solution.

Leaching operations carried out with the inhibited leaching solutions of the present invention will generally be carried out at room temperature or thereabouts. This temperature, however, is largely a matter of convenience 1. A method of inhibiting corrosion of metallic titanium by acids which comprises incorporating into said acids a corrosion-inhibiting concentration of a nitro-substituted organic compound.

2. A method for inhibiting corrosion of metallic titanium by acid solutions which comprises incorporating into said solution a corrosion-inhibiting concentration of at least 0.01 weight percent of a nitro-aromatic compound.

3. A method of inhibiting corrosion of metallic titanium by aqueous acid solutions which comprises incorporating into said solution a corrosion-inhibiting concentration between about 0.05 weight percent and about 5 weight percent of a nitro-substituted aromatic hydrocarbon com- .pound.

4. A method of inhibiting corrosion of metallic titanium by aqueous acid solutions which comprises incorporating into said solution a corrosion-inhibiting con- References Cited in the file of this patent UNITED STATES PATENTS Cantrell et al Oct. 25, 1938 6 Routson Nov. 26, 1946 Robinson Oct. 5, 1948 Broyles May 26, 1953 Newschwander Dec. 15, 1953 Hayes et a1. Nov. 9, 1954 McKinley Apr. 26, 1955 OTHER REFERENCES Bockris et al. in Chemical Abstracts, vol. 43, col. 952(b), 1949.

Cobb et al., article, Resistance of Titanium to Sulfuric and Hydrochloric Acids Inhibited by Ferric and Cupric Ions, in Journal of the Electrochemical Society, I anu- Whaley Jan. 2, 1940 5 ary 1952, pp. 13-15. 

5. A METHOD OF LEACHING A WATER-SOLUBLE INORGANIC SALT FROM A MIXTURE OF SAID SALT AND METALLIC TITANIUM, WHICH METHOFD COMPRISES CONTACTING SAID MIXTURE WITH AN ACIDIC SOLUTION CONTAINING A CORROSION-INHIBITING CONCENTRATION OF NITORBENZENE TO DISSOLVE SAID SALT IN SAID SOLUTION, AND THEREAFTER SEPARATING THE METALLIC TITANIUM FROM SAID SOLUTION. 